How do I know if I need a new battery for my phone?

It is kind of up to you how much you can tolerate frequently recharging your device. Apple provides guidelines about battery health here --> iPhone Battery and Performance - Apple Support That includes a message about when it thinks it necessary to change your battery.

Baseball2021 wrote:

How do I know when I need a new battery for my phone?

Go to Settings/Battery/Battery Health. You need a new battery if:

• The battery condition is under 80%

OR

• There is a message under “Peak Performance” that says the battery is degraded

When the battery health drops to 80%.